West German Offenlegungsschrift No. 3,537,704 discloses a furnace boiler, in which a blowpipe with a horizontally directed flame burns in a firebox, which is oriented horizontally and which is open over its entire length and width along its lower side, which is essentially parallel to the axis of the flame of the blowpipe, toward a flue gas collection space containing heat exchangers. The walls of the firebox are cooled by a water jacket, through which the utility water to be heated and the water of the heating system flow. The water jacket which cools the walls of the firebox takes up most of the heat generated by the burner, whereas the heat exchangers installed in the adjoining flue gas collection space act only as a secondary heating surface, which cool the flue gases down to a temperature of about 160.degree.-180.degree. C., at which temperature the flue gases enter the chimney.
The large volume of the water jacket surrounding the walls of the firebox cools these walls intensively. The blowpipe must, therefore, be operated at a constant burner output, which is coordinated with the cooling of the firebox wall. Reducing the burner output, e.g., to adapt to a lower heating need between seasons, would lead to the overcooling of the flame, which results in a high pollutant level in the flue gases and even to condensation on the firebox wall. The output of the blowpipe can, therefore, not be reduced to adjust to reduced heating needs. The burner is instead operated intermittently at maximum output. This intermittent operation means in turn that the burner has to be started up frequently, and each time this happens the entire volume of the water jacket cooling the firebox wall must be heated up again. In this heat-up phase, the burner flame is always overcooled, with the result that the pollutant level increases and efficiency declines.
To prevent the disadvantages of the overcooling of the flame, preferably so-called combustion aids are installed in small furnace boilers of low output up to about 40 kW. The firebox wall enclosed and cooled by the water jacket consists of a cylindrical casting, in which a high-grade steel pipe is inserted coaxially. The burner flame burns inside this pipe. The high-grade steel pipe is held in place a certain distance away from the walls of the casting by inward-facing ribs on the walls. The hot combustion chamber formed by the high-grade steel pipe is, therefore, in practice not cooled. Because of the low heat capacity and the absence of cooling, this combustion chamber reaches a high temperature very quickly when the burner is started, which means that the fuel is burned without residue not only during continuous operation but also very soon after the burner has been started. Only while the combustion gases are flowing between the combustion chamber and the cast wall of the firebox is any heat extracted from them.
In this furnace boiler, the high temperature in the hot combustion chamber and the long residence time of the combustion gases in the hot combustion chamber convert a large amount of the nitrogen from the atmosphere into NO.sub.x, which means that the waste gases contain a large percentage of harmful nitrogen oxides. In spite of its ribs, the firebox wall surrounding the combustion chamber coaxially cannot extract enough heat from the hot flue gases. For this reason, it is usually necessary to provide secondary heating surfaces to achieve a satisfactory degree of efficiency. These make the design of the furnace boiler more complicated. The firebox wall, which usually consists of cast iron, and the surrounding water jacket have a high heat capacity, which means that the furnace boiler has a large amount of inertia. The heat capacity is influenced in particular by the fact that the firebox must have a large volume so that it can hold the hot combustion chamber used and provide a sufficiently large heat exchange surface.
From West German Pat. No. 3,205,121, finally, it is known that the wall of the firebox of a furnace boiler can be made of a double layer of sheet metal, the inner and the outer walls touching each other only at certain points to thereby reduce the heat transfer from the inner to the outer walls. The inner wall thus reaches a relatively high temperature, whereas the outer wall is cooled by the water jacket of the furnace boiler surrounding the wall coaxially.
The reduction of the heat transfer from the inner wall to the outer wall is problematic in this furnace boiler, because it is impossible to heat the water jacket cooling the outer wall economically when the heat transfer is too low; whereas, when the heat transfer is too high, the inner wall and thus the flame, become overcooled, with all the attendant disadvantages described above. Because the firebox walls serve to transfer heat to the heating water, this furnace boiler can also be operated only at an essentially constant burner output. Reducing the burner output to adjust to a lower heat requirement is impossible especially during continuous operation, because the flame becomes overcooled at the cooled firebox wall.